This invention relates to the general domain of aerodynamics and concerns digital simulation of aerodynamic flows for an aircraft.
Its application lies in the aeronautic field for which the design of an aircraft requires precise knowledge of aerodynamic coefficients associated with its various elements.
When designing an aircraft, an attempt is made to determine global aerodynamic coefficients associated with its various elements, for example coefficients associated with the wings such as lift, drag and the pitching moment.
These coefficients can be determined in different ways, particularly by digital simulation of fluid flows that consists of analysing movements of a fluid or the effects of these movements by digital solution of equations governing the fluid.
A digital model is chosen to reproduce the fluid flow in a zone in space that surrounds an element of the aircraft. This zone in space is called the geometric domain of the fluid or calculation domain.
Digital simulation is used to determine physical values (for example speed, pressure, temperature, density, etc.) for each point in the calculation domain, for a global cost usually much lower than wind tunnel or flight tests.
The equations to be solved can be very varied depending on the chosen approximations that are usually the result of a compromise between the need for a physical representation and the calculation load, the equations most frequently used being Euler equations (representing a non-viscous adiabatic fluid) and Navier-Stokes equations (representing a viscous heat conducting fluid). Navier-Stokes equations are usually averaged and complemented by turbulence models.
These equations are digitally solved by computers, using meshes discretising the geometric domain of the fluid to be studied and digital schemes that replace the continuous form of equations by discrete forms. This solution is usually made iteratively, in other words starting from an initial state (for example corresponding to a uniform flow) and performing successive calculation iterations consisting of calculating the next state from the current state.
Ideally, this iterative method should lead to a state that no longer changes as more iterations are carried out and corresponds to a rigorous solution of discretised equations. In practice, this state is not achieved regardless of the number of iterations made and the simulation has to be stopped as a function of specific criteria, for example after reaching a number of iterations fixed in advance or when the difference between two successive states is less than a given quantity.
The convergence quality of a digital simulation of aerodynamic flows can be evaluated based on plots of changes to aerodynamic coefficients made at a linear scale. The values of an aerodynamic coefficient can be positive, negative or zero and their convergence towards a previously unknown value is studied.
The use of a linear scale for the plot of the change to aerodynamic coefficients combined with the fact that this change normally converges, results in a plateau being obtained on the plot of the curve, which is interpreted as a demonstration of convergence.
Nevertheless, with this type of plot, it is difficult to see precisely how these coefficients change in the plateau, which makes a precise analysis of the simulation convergence more difficult.
Zooms of the plot then have to be made frequently, but this has the disadvantage of requiring manual work to use plotting software and a large calculation workload. Furthermore, these zooms are impossible if all that is available are plots printed on paper.
The purpose of this invention is to disclose a method for simulating fluid flows to determine changes to aerodynamic coefficients correcting the above-mentioned disadvantages.